In the production of paper and board and more specifically the fibrous web is formed by draining water from an aqueous suspension of pulp and fillers in the forming section of a paper machine. In this process it is desirable to drain the water as fast as possible and to retain on the forming fabrics the largest possible proportion of pulp and filler. In the absence of retentions additives a substantial portion of pulp fibres and various fine components of the production suspension are not retained on or between forming fabrics but pass through them and leave the former with so called whitewater.
During the fabrication of paper, a flat jet of a dilute suspension of pulp and additives is injected on to the surface of a specially designed textile called a forming fabric, or into a converging gap formed by two forming fabrics. The function of the forming fabric is to allow rapid drainage of water, while retaining the largest possible fraction of solids from the suspension. The bulk of the water is rapidly drained through one or two fabrics, while a substantial portion of the suspended solids, such as fibres, fines and filler, is retained by the fabric in the form of a sheet. Ideally, all the solid material dispersed between the fabrics would be retained in the sheet; however, a portion of the solids, especially very small particles and colloidal substances, escapes through the interstices in the forming fabrics. Retention is thus always less than perfect and for a majority of paper grades made from mechanical pulps, it usually varies between about 20% and 60%. When the retention is poor, a large amount of material must be re-circulated to obtain a sheet with the desired basis weight and some of this material is eventually lost to the effluent treatment.
Only a fraction of a second is usually available for water drainage on rapidly-operating, modern paper machines and, therefore, for good machine operation it is critical that the drainage occurs rapidly. However, during rapid drainage a high shear stress occurs in the forming zone, which tends to separate the particles of fillers and fines from the fibres, and thus impair their retention.
The primary component of mechanical pulps is fibres, but the pulps also contain about 30% of small wood debris usually referred to as fines. Furthermore, mineral pigments of small particle size are often used as fillers, in amounts ranging from a few % to over 40% of sheet mass. These fillers are added to improve the printing quality of the paper, and to reduce its cost. The fines and fillers are too small to be retained on the forming fabrics by filtration. In the absence of chemical additives (commonly known as retention aids), a large proportion of these materials passes through the forming fabric and re-circulates in the white water loop. Poor retention thus causes the loss of valuable papermaking material, impairs product quality and increases the cost of both production and waste water treatment.
In a common papermaking practice, polymeric materials (usually called retention aids) are added to the papermaking furnish in order to improve the retention of fines and fillers. The retention aids are adsorbed on to the surface of fines and fillers causing the agglomeration of fine particles into flocs and, eventually, their adsorption on to the surface of pulp fibres. Polymeric additives described in the literature and available from various suppliers are used alone or with small molecular weight co-factors, sometimes also with one or two additional polymeric components, or with organic or inorganic micro particle materials.
Pulp fibres and fines, and also most fillers, are negatively charged. Many retention aids are positively charged polymers which are adsorbed on to the negatively charged fibres via electrostatic interactions. Electrostatic mechanism of retention can be efficient for chemical pulps, which are composed of relatively pure cellulose, as most of the lignin and hemicelluloses originally present in the wood are eliminated during pulping and bleaching. By contrast, mechanical pulps contain almost all of the original wood mass, including almost all the hemicelluloses and lignin. Compared with cellulose, these non-cellulose wood components usually carry a much greater negative charge. Because of the very large specific surface of mechanical pulp, a large amount of negative charge is thus available for interaction with added cationic polymers. Negative charge also resides on the dissolved and colloidally dispersed wood components which are present in the suspensions of mechanical pulps. In total, the high negative charge residing on mechanical pulps overwhelms the positive charges found on common retention aids, and greatly diminishes their efficiency.
The cationic charge of many papermaking polymers is due to the presence of quaternary amino groups, which remain cationic at all values of solution pH, or as tertiary amino groups which are cationic only in acidic solution, where the tertiary amino groups are protonated. Some water-soluble polymers have a high density of cationic charge and are designed to reduce the negative charge of papermaking furnishes. One example of such polymers is poly(diallyl-dimethylammonium chloride), known as polydadmac [D. Horn and F. Linhart, in Paper Chemistry, Ch. 52nd Ed. by I. C. Roberts, Blackie Academic and Professional, London 1996.].
Other water-soluble polymers such as cationic starches [U.S. Pat. No. 2,768,162 (1956)] have only a low cationic charge, which improves their retention in the fibrous sheet, although the charge might not be sufficiently high to make these polymers act as good retention aids. Dry strength additives are often used to increase the strength of dry paper and board; cationic starches and water-soluble synthetic polymers such as polyacrylamides are examples. Cationic starches are the most commonly used dry strength additives. They are obtained by substituting natural starches with substituents containing tertiary or quaternary amino groups. Numerous patents exist on the preparation and application of various cationic starches. As examples there may be listed the following patents, H. Dreyfus [German patent 550,760 (1929)], M. Hartman [U.S. Pat. No. 1,777,970 (1930)] C. P. L. Vaughan [U.S. Pat. Nos. 2,591,748 (1952) and 2,623,042 (1952)], P. Schlack [U.S. Pat. No. 2,131,120 (1938)], C. L. Hoffpauir and J. D. Guthrie [Textile Res. J., Vol 20, page 617 (1950),] and E. F. Evans [U.S. Pat. No. 2,768,162 (1956)]. Conversion of neutral starch to cationic starch increases the cost of this product, and also reduces its molecular weight causing a loss of yield and reduction of some desirable properties. In contrast to polymers containing primary amino groups such as polyethyleneamine and chitosan, the conventional cationic starches containing tertiary or quaternary amino groups cannot form imino groups with aldehydes.
Wet strength additives are well known, and have been extensively described in the literature. Two types of chemical agents are used to improve the wet strength of paper, namely those that impart permanent wet strength and those that provide temporary wet strength. The common “permanent” wet strength agents (often thermosetting resins) are cationic epichlorohydrin-based polymers. Papers made with these resins retain a substantial portion of their dry strength even when soaked in water for a long time. The cationic epichlorohydrin-based resins, such as polyamide-epichlorohydrin resin, polyamine-epichlorohydrin resin and polyamide-epichlorohydrin epoxide resin represent about 94% of the total wet-strength chemicals market. The most effective pH range for these resins is 6.5-8.5. Papers made with high dosage levels of cationic polyamide-epichlorohydrin (PAE) resin, such as Kymene 557H (trademark) (from Hercules) and Amers 8855 (trademark) (from Georgia Pacific), can have improved dry strength and permanent wet strength. The permanency of wet strength developed with PAE resin results from the formation of water-resistant chemical bonds within the paper structure. When paper containing such resin is heat dried, the reactive group of the resin, hydroxyazetidinium, bonds with the amino group of the resin itself, as well as with the carboxyl groups on the fibres. One major problem associated with use of high dosage levels of permanent wet strength resins is that the broke can be difficult to repulp.
It is widely accepted that the temporary wet strength agents introduce into the fibre network covalent bonds which slowly react in water and eventually are disrupted. The chemical agents that can be used to impart temporary wet strength may include glyoxal monomer (CHOCHO), glyoxalated synthetic polymers such as polyacrylamide (Parez 631 NC and Parez 75A—trademark from Bayer), aldehyde starches such as cationic dialdehyde starch (CoBond 1000 —trademark from Hercules) produced by oxidation of the glucose units of starch, and cationic aldehyde starch DAS (made by substitution of hydroxyl groups on the glucose units of starch), and polymers containing primary amino groups such as polyethylenimine, polyvinylamine and chitosan. The aldehyde groups form hemiacetal bonds with the hydroxyl groups of cellulose, whereas the primary amino groups may react with aldehyde groups and carboxyl groups of cellulose to form imino bonds and ionic interactions.
Wet-web strength additives are the products that are capable of increasing the strength of a freshly-formed, never-dried wet web as it proceeds from the wet end of a paper machine towards the dryer section. These products are new in the industry and are not widely used. Wet-web strength additives have been described in the literature, namely chitosan, polyethylenimine, cationic aldehyde starch and glyoxylated polyacrylamide.
Starches substituted with primary amino groups have been prepared using complicated procedures, which, if applied on a commercial scale, would make the products too expensive and therefore unsuitable for application as papermaking additives. Examples of such synthetic routes are described by F. Pancirolli and A. A. Houghton [UK patent 493,513 (1938)]. An alternative route for the production of starch additives containing primary amino groups was published recently [M. Antal, et. al., U.S. Pat. No. 6,455,661, (2002)]. At this time this invention did not yet find a commercial application.
Polyethyleneimine is a commercially-used, water-soluble, cationic, papermaking additive, which contains a certain proportion of its amino groups in their primary form. It has been reported that chitosan is an efficient retention additive and strengthening agent for mechanical pulps. These results were described, for example, in [M. Laleg and I. I. Pikulik, Nordic Pulp and Paper Res. J., Vol. 7, No. 4 page 174 (1992)].
Chitin is a natural polysaccharide with a structure similar to that of cellulose, but differs from cellulose in that one hydroxyl group in every glucose unit of chitosan is replaced by an acetylamino group. Chitosan is produced from chitin by deacetylation of amide groups. This reaction is usually carried out with a large excess of concentrated sodium or potassium hydroxide at high temperature. The chemical structure of chitosan resembles that of cellulose, but differs from cellulose in that one hydroxyl group in every glucose unit of chitosan is replaced by an amino group.
In acidic solutions, amino groups of chitosan become positively charged, making chitosan solutions strongly cationic. Therefore when added to the papermaking furnish, chitosan quickly absorbs on negatively charged fibres and fines. It is believed that the primary amino groups of chitosan can react with the carbonyl groups that are found in large amounts especially in the lignin components of mechanical pulps. The product of such reaction is a strong, chemical imino bond between the fibre or fine and the additive. Since each polymeric molecule of the additive can form similar bonds with two or several fibres, the entire fibrous network can be cross linked and reinforced by a polymer that contains primary amino groups. This cross linking can occur during the consolidation of wet web and thus polymers containing primary amino groups can increase the strength of never-dried wet webs. The strength of wet webs is critical for good operation of paper machines especially at high speed.
The shells of sea crustaceans are the most common source of chitin for chitosan production. The procedure for preparation of chitosan from this source is complex, requiring a large amount of chemicals, and the yield is only about 20% based on dry shells. Therefore, the cost of chitin produced from shellfish is high and chitosan is relatively expensive. Because of its high cost the application of chitosan in papermaking has only been sporadic and has not found acceptance in any paper mill at the present time. The world supply of sea shells suitable for industrial production is limited, and chitosan from this source could not satisfy a large scale demand from the paper industry. Thus, a new class of retention aids, which would have the chemical nature and mechanism of action of chitosan, but which could be produced in large quantities at low cost would be highly desirable.
Several procedures have been described for production of chitosan from microbiological materials. Some microorganisms, for example such moulds as Aspergilus Niger, Mucor mucedo, or Penicillium, contain chitin as a part of their cell wall, but generally these chitins are chemically attached to carbohydrate polymers. The published or patented procedures for the extraction of chitosan or chitosan-containing materials are aimed at product used in medicine, cosmetics, food industry, for extraction of heavy metals or for other high-value products. Therefore production procedures that use several reaction steps, large amount of chemicals or reactions in pressurized vessels are acceptable. The products are usually pure forms of chitosan in which the deacetylation equals or exceeds 85% and from which most of the carbohydrates have been removed. For example, DE 29 23 802 A1 disclosed the treatment of the mould Mucor rouxii with 40% sodium hydroxide at 128° C. To prevent boiling, such a reaction needs to be carried out in a pressurized reaction. The product was capable of binding heavy metals. According to WO 2003086281, chitosan can be prepared by reacting dry Aspergillus Niger mycelium with a solution of sodium hydroxide at 110° C. Also, WO 2001068714 (US 2002025945) describes a method for producing at least 85% deacetylated chitosan by reacting the chitin-containing biomass with 25% alkali at 95° C. for at least 10 hours. The pure chitosan is then separated from the alkaline solutions and washed. This procedure is preferably preceded by a pre-treatment of biomass with alkaline solution. The product could be used for cosmetic, medical and dietary application.
U.S. Pat. No. 3,632,575 describes the production of chitin-containing material for healing of wounds. The production involves the extraction of biomass with chloroform, an 18 h reaction with a solution of sodium hydroxide, acidification with hydrochloric acid and purification by dialysis. U.S. Pat. No. 6,333,399 describes production of chitosan-glucan complexes for application in medicine, food industry or in environmental protection. The production of these materials involves treatment of Aspergilus Niger in at least four reaction steps:                1. Treatment with alkaline solution to remove proteins and glucan complexes; this procedure is preferably repeated two to four times, followed by filtration of solids;        2. Acidification with a mineral acid to remove mineral materials;        3. A second treatment with alkaline solution at temperatures between 90 and 150° C. to hydrolyze chitin to chitosan;        4. Treatment with low concentration mineral acid.        
This procedure requires large amounts of reagents, a pressurized reaction vessel and a relatively large amount of time, which inevitably increases the cost of final product. Such a procedure might be convenient for a product used for medical purposes or extraction of heavy or radioactive metals but would be too expensive for a papermaking additive.
The common feature of the described methods for the production of chitosan from microbiological sources is the complex and expensive production method of preparation which is suitable for the products that are used in cosmetics, medicine or the food industry. The chitosan is isolated from the reaction mixture and is used in a relatively pure form.